Plastic net composed of co-extruded composite strands

ABSTRACT

An improved plastic net product and its method of manufacture. Intersecting strands of plastic are extruded to form a net structure. Each strand is comprised of a plastic core of a first polymer sandwiched between layers of another polymer, all of which are co-extruded.

This is a division of application Ser. No. 593,829, U.S. Pat. No.4,656,075 filed Mar. 27, 1984.

BACKGROUND OF THE INVENTION

This invention relates to the manufacture of plastic net products and inparticular to an improved plastic net product and method formanufacturing same.

The continuous extrusion of plastic net started in about 1956 with theprocess described in the Mercer U.S. Pat. No. 2,919,467. Since thattime, many patents have issued in the United States as well as in othercountries describing improvements and refinements in the continousextrusion process.

The initial extrusion process has developed along two basic lines: thefirst, in which plastic sheet is extruded and holes are formed thereinto provide a net-like structure, and the second, in which individualplastic strands are extruded in an interconnecting network to providethe net-like structure. This invention is specifically concerned with avariation in the latter of the two.

Methods for practicing the latter technique are well known. Forinstance, U.S. Pat. Nos. 3,700,521; 3,767,353; 3,723,218; 4,123,491;4,152,479 and 4,190,692 show apparatus and methods for making net bycontinuous extrusion of strands.

The disclosures of the above-mentioned issued patents are incorporatedby reference into the present specification as are all of the patentswhich may be referred to hereinbelow in further description of thisinvention.

In all of these patents at least one set of strands is extruded througha plurality of spaced individual orifices. A second set of strands inthe net structure may be extruded through a second set of spacedindividual orifices or a second set of strands may be preferentiallyextruded periodically through a continuous annular orifice slit. In allcases the two sets of strands are extruded such that the individualstrands intersect at an angle and form integral joints in the extrudedplastic net. The resulting extruded flat sheet in one process or tube ofplastic net in another process is cooled to set the plastic in thestrands, as for example in a water bath, and the net is drawn away fromthe extrusion orifices by nip rolls or other suitable drawing means.When a tube of net is extruded, it is usually drawn over a cylindricalmandrel which may stretch the strands and enlarge the openings in thenet structure. Such stretching of the strands over the mandrelpreferentially orients the plastic but in practice the net ischaracterized as being "unoriented".

For many applications, it is desirable to further stretch the netstrands and more fully orient the plastic and this may be done, where asin the case of a tube of extruded net, the tube is heated and stretchedlongitudinally to further elongate and orient the strands. Stretchingthe tube causes it to collapse while the tube is being stretchedlongitudinally. If the tube has been slit and formed into a flat sheetof extruded net, the flat sheet may be heated and one set of strands maybe stretched and oriented in one direction, and in a second separatestep, the second set of strands may be stretched to orient the strandsin a second direction. Some plastic net may be oriented at roomtemperature but as a practical matter the net is heated to speed up andfacilitate orientation of the net.

A significant problem in the manufacture of high temperature orientednetting such as nylon or other polyamides and polyester netting is thatsuch materials have a low-melt-strength and degrade easily. Thelow-melt-strength of such polymer resins makes it difficult to produce auniform extruded net and also makes it difficult to produce a qualityoriented product.

Also, while the extrusion process works very well for the extrusion ofnon-polar polyolefins such as polypropylene, other resins which arepolar such as the aforementioned nylon or other polyamides, and otherlow-melt-strength resins tend to stick to the extrusion die. Thedegradation of molten resin during extrusion is a problem becausedegraded material builds up on the die lips. This makes an equipmentshut down necessary approximately every six-eight hours to clean the dieparts and interrupts the continuous process.

Both low-melt-strength and resin degradation have been found to presentmajor problems in the extrusion of net-like products of nylon, polyesterand the like.

This invention has as an objective, the successful and continuousextrusion of low-melt-strength polymer net product without die build-up.

Also, since fusible net products are being used in greater and greaterquantities as a reinforcing structure in fabric-to-fabric laminates suchas paper toweling, reinforced tissue and the like, the net product ofthis invention may be used to improve these laminates. In some forms ofthe invention, the net product may also utilize a co-extruded outerlayer of heat sealable polymer carried by a high temperature core as afusible adhesive for heat bonding or laminating such tissues, fabricsand the like, together with the reinforcing net product sandwichedtherebetween. For such uses, the extruded fusible net product of thisinvention has, among other advantages, that of uniformity, dimensionalstability and high strength as compared to other reinforcing products.

It is, therefore, another object of this invention to provide animproved co-extruded net product which not only functions to reinforcesuch laminates but also provides the adhesive necessary to hold thelaminate together.

These and other objects of the invention will be apparent from thedescription provided hereinbelow.

SUMMARY OF THE INVENTION

In accordance with this invention it has been unexpectedly found thatindividual interconnecting strands of polymer individually extruded toform net-like structures can be co-extruded in the form of compositelaminar strands. As provided by the invention, laminar flow of aplurality of polymers, including copolymers, through a coextrusion dieto form individual strands has been found, contrary to expectation, toresult in extruded composite strands which exhibit laminar structure. Itwas fully expected to the contrary that extrusion of laminar flowthrough the relatively small strand orifices of the die coupled with theshear action created when intersecting strands are formed by the diewould result in disturbance of the laminar flow and mixing of theplurality of polymers to provide a strand of mixed polymers without adistinguishing laminar structure, particularly at the strandintersections or joint areas.

In one preferred form, the invention provides a net-like structurecomposed of individually extruded strands, each strand of which isformed of a low-melt-strength polymer sandwiched between outer layers ofa more stable polymer material as each strand emerges from athree-layered die. Three-layered strands are referred to herein as being"tri-extruded".

A more uniform extrusion results when the low-melt-strength resin isencapsulated between two high-melt-strength resins. Typicalhigh-melt-strength resins are polypropylene, high-density polyethylene,low-density polyethylene and linear low-density polyethylene. Polyesteris a preferred high temperature resin because of its relatively low costand low orientation temperature. The encapsulation of polyester or thelike between two high-melt-strength resins such as two polyolefinsprevents build up of degraded resin at the die lips. The reduced buildup minimizes extrusion down time and makes the operation moreeconomical. The term polymer is used herein in a general sense toinclude various copolymers as well.

The composite net-like product of the invention may be oriented eitheruniaxially or biaxially in known manner. Even orientation of"tri-extruded" (referring to three laminar layered strands) net, such aspolyester between polyolefins (a preferred form of the invention) doesnot pose a problem since polyester is oriented at about 200°-220° F.which is lower than the melting point of most polyolefins.

In another preferred embodiment of the invention, a net product isprovided in which the outer polymer on the strands is heat sealable.

The co-extrusion and specifically tri-extrusion of a net-like product inthe film form is known and described in U.S. Pat. No. 4,410,587.However, this patent does not suggest that laminar flow of polymersthrough a co-extrusion or tri-extrusion die can be accomplished toprovide individual, composite, laminar extruded strands.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details of the invention and the preferred type of extrusionapparatus used in the process may be readily understood by reference tothe following description and the drawings in which:

FIG. 1 is an elevational cross section on the vertical axis of a portionof an extrusion die;

FIG. 2 is an enlarged fragmentary detail of part of the extrusion die ofFIG. 1 taken in the area marked 2 in FIG. 1;

FIG. 3 is a pictorial view of the preferred product, a tubular extrudednet, with a small part of the netting cut on the longitudinal axis toillustrate how the tubular netting may be cut and made to open up into aflat sheet of netting;

FIG. 4 is a fragmentary view of a small portion of the flattened nettingdescribed in FIG. 3 in enlarged scale;

FIG. 5 is a sectional view of a strand of the netting taken along line5--5 of FIG. 4 and greatly enlarged;

FIG. 6 is a fragmentary to plan view of the die head plate of theextrusion head only;

FIG. 7 is a fragmentary section in enlarged detail of part of FIG. 1taken in the area marked 7 of FIG. 1 with the die striker in a loweredposition;

FIG. 8 is a fragmentary view similar to that of FIG. 7 but with thestriker in a raised position, and

FIG. 9 is a pictorial view of a form of the netting of the inventionused as a sandwiched reinforcing element in a piece of reinforcedtissue.

DETAILED DESCRIPTION OF THE INVENTION

Generally, in accordance with the invention, plastic machine-directionstrands are continuously extruded through a plurality of spaced orificeopenings which are annularly disposed about an extrusion die head. Anadjacent continuous orifice extending annularly about the die head isalternately covered and uncovered so as to provide, each time it isuncovered, an interconnecting transverse-direction strand. The mechanismwhich is moved to alternately cover and uncover the continuous annularorifice is referred to herein as a striker.

The extrusion produced by the above structure comprises a tube ofplastic net which in general has rectangular-shaped openings extendingalong the length of the tube.

As is also known in the prior art, slitting the tube at the end remotefrom the extrusion die allows the net-like product to be flattened intoa sheet-like form which may then be subjected to orientation proceduresand other treatment as is well known in the art.

Turning now specifically to the drawing Figures, preferred embodimentsof the invention will be described in detail as involving thetri-extrusion of a network of composite strands each of which iscomprised of a laminated, extruded structure exhibiting a distinct innerlayer sandwiched between oppositely disposed outer layers of polymermaterial. It is to be understood, that the invention is also applicableto co-extruded forms generally such as a two-layer type. However, itwill be described with respect to the three-layer or tri-extrudedpreferred type.

Referring to FIG. 1, an annular extrusion die 10 is shown in part asincluding an annular extrusion head 12. Die 10 is of the type generallyknown to the art and will include standard features such as supportbolts and adjustment structures (not shown) as are known and familiar.However, the die differs in one important respect. In die 10, threeseparate reservoirs 18, 20 and 22 of polymer materials 18a, 20a and 22a(shown in FIG. 2), respectively, are maintained under pressure andprovided to the die by an extrusion system (not shown) and individualpolymeric material flow is supplied to die 10 as indicated to theannularly disposed reservoirs 18, 20 and 22. All of these reservoirscommunicate and direct flow into a common annular reservoir 24.Reservoir 24 receives the three separate flows of polymer materials from18, 20 and 22, respectively, which come together in annular reservoir 24in a three-layer laminar flow pattern which exits from reservoir 24 intoa common annular feed channel 26. This is best seen in FIG. 2 whichshows three laminar layers of flowing polymer 18a, 20a and 22a. Thethree-layers flow past the die lips, generally indicated at 28, to formcomposite three-layer strands in an interconnecting network whichprovides the familiar tubular net-like structure 50 shown in FIGS. 1 and3.

For the purpose of extruding machine direction strands 30 and transversedirection strands 32, lips 28 of the die are preferably formed as shownand include on the upper die head plate surface 14 a series of raisedand spaced lands 34, best seen in FIG. 6, which form therebetween aseries of annularly positioned lower die orifices 36 through which themachine-direction strands 30 are continuously extruded from the die.

As can be seen best in FIG. 1, lands 34 on die head plate 14 are spacedfrom portion 38 of die 10 so as to provide an upper passage 40 above thelands in the form of a continuous annular orifice which extends aroundthe die. Associated with upper passage 40 is a reciprocable striker 42which may be alternately lowered and raised by a suitable meansindicated at 44 to cover and uncover passage 40 (shown uncovered in FIG.1). Striker 42 contacts the upper surface of lands 34 on its downwardstroke as shown.

When striker 42 is in the lower position best seen in (FIG. 7), passage40 is closed and only the continous extrusion of machine-directionstrands 30 occurs through lower orifices 36 which remain open at alltimes to continuously extrude the spaced plurality of machine-directionstrands 30 in annularly distributed pattern as shown in FIGS. 1 and 3.When striker 42 is raised to its upper position (best seen in FIG. 8,passage 40 is uncovered to allow the extrusion of an annulartransverse-direction strand 32.

Reciprocating movement of striker 42, which is relatively rapid,provides for the spaced extrusion of transverse-direction strands 32 tointerconnect the continuously extending machine-direction strands 30thereby forming the tubular net-like structure 50 shown in FIGS. 1 and3.

The resulting tube of net generally indicated at 50 is drawn downwardlyaway from the extrusion die head 12, preferably over a cylindricalmandrel (not shown) and through a water bath (not shown) preferably by apair of nip rolls (not shown), as is already known in the art. After theplastic strands have set, the tube of net is preferably slitlongitudinally in the known manner as generally indicated at 52 in FIG.3 and opened into a flat sheet 54, a fragment of which is shown in FIG.4. The sheet may be accumulated on a wind-up roll in the known manner(not shown).

In accordance with the present invention, the strands extruded with thedie of FIG. 1 are composite and exhibit a laminated multi-layerstructure as is shown in FIG. 5 when polymer 20a differs from polymers18a and 22a. The multi-layer strucuture shown is the preferredthree-layer tri-extruded structure comprising a core polymer 20ssandwiched between a first outer layer 18a and a second outer layer 22a,which are generally oppositely disposed relative to each other.

The unexpected advantage provided lies in the fact that the compositelaminar structure provided by the three-layer flow within the die bodyis maintained and exhibited in the extruded strands even thoughextrusion is accomplished through the relatively small strand-formingorifices formed in extrusion head 12 coupled with the shearing actionprovided by the striker 42.

In accordance with a preferred embodiment of the invention, the outerpolymer layers 18a and 22a may be relatively stable, high-meltingstrength polymers, which may be chosen from the group of polyolefinresins. These resins include polypropylene, high-density polyethylene,low-density polyethylene, linear-low-density polyethylene andco-polymers of this group. Layers 18a and 22a may be of the same polymeror different polymer, such as EVA and EMA. A preferred resin for bothlayers is low-density polyethylene. A more preferred high-melt strengthpolymer for both layers is polypropylene.

The core polymer material 20a may be chosen from those polymers whichare generally described herein as being of the low-melt strengthpolymers and may be chosen from the group consisting of nylon and otherpolyamides, polycaprolactones, polyesters and co-polymers of this group.

It will be appreciated that the invention is not limited to theaforementioned groups of polymers, copolymers, and the like and thatthere exists a considerable number of polymers which will fall into thelow-melt strength category and the high-melt strength category,relatively speaking.

In the tri-extruded form, a preferred combination comprises an innercore of polyester layer 20a encapsulated or sandwiched between outerlayers 18a and 22a of polypropylene.

In another preferred embodiment of the invention (not utilizing theformer polymer combination of a low-melt-strength inner core and ahigh-melt-strength outer polymer) a preferred combination comprises acore layer 20a of polypropylene while the two outer layers 18a and 22acomprise ethylene-methyl acrylate copolymer (EMA), ethylene-vinylacetate copolymer (EVA), or an ionomer resin such as Surlyn®, a modifiedpolyethylene marketed by E. I. DuPont de Nemours Co. the former beingmost preferred. This particular combination of materials when biaxiallyoriented provides a tri-extruded reinforcement net product particularlyuseful for tissue reinforcement when sandwiched between two layers ofouter covering material such as paper, tissue, foil or the like asschematically indicated in FIG. 9 to form a composite reinforcedlaminate. The outer covering layers are indicated at 60 and 62, thereinforcing net structure being indicated at 64. The ethylene-methylacrylate copolymer (EMA) has the advantage of providing a heat fusibleadhesive function which heat bonds the reinforcing net-like structure tothe outer fabric layers 60 and 62 when the composite is subjected toheat and pressure.

It will be understood that the claims are intended to cover all changesand modifications of the preferred embodiments of the invention hereinchosen for the purpose of illustration.

What is claimed is:
 1. A method of manufacturing plastic net comprisingthe steps:(a) coextruding a laminar composite of at least differentkinds of plastic in superimposed bonded relationship to each otherthrough a plurality of individual openings in a die to form a firstplurality of composite plastic strands; (b) coextruding a laminarcomposite of at least two different kinds of plastic in superimposed,bonded relationship to each other from a die to form a second pluralityof composite plastic strands to intersect the first plurality of strandsthereby forming a net-like structure.
 2. In a method of manufacturingplastic net-like structure in sheet form by melt extrusion of theplastic from a die apparatus, the net being formed by extruding two setsof spaced strands of the plastic and in which the first set of spacedstrands is extruded in a spaced circular arrangement while the secondset of spaced strands is extruded substantially transverse and normal tothe strands of the first set to form in combination therewith thenet-like structure in tubular form which is then slit to form a sheet inwhich the strands of the first set are aligned substantially parallel tothe length of the sheet and the strands of the second set are aligned atsubstantially a right angle to the strands of the first set, theimprovement comprising the coextrusion from the die apparatus of alaminar composite of at least two different kinds of plastic materialmaking up the strands, the coextrusion being accomplished byestablishing separated, annularly distributed flow patterns of thedifferent plastics within the die apparatus and bringing them togetherin superimposed laminar relationship to each other within the dieapparatus prior to their coextrusion from the die apparatus whereby theresultant strands have a laminar structure in cross-section.
 3. Themethod fo claim 1 wherein three layers of polymers are coextruded toform each of the strands, one layer comprising a first kind of polymersandwiched between the other two layers which comprise a second kind ofpolymer.
 4. The method of claim 3 wherein the first kind of polymer isone of relatively low melt strength and the secoond are of relativelyhigh melt strength.
 5. The method of claim 3 wherein the first is apolyester and the second is a polyolefin.
 6. The method of claim 4wherein the first kind of polymer is selected from the group consistingof polyamides, polyesters and copolymers of this group.
 7. The method ofclaim 6 wherein the polyamide is nylon.
 8. The method of claim 4 whereinthe second kind are selected from the group consisting of polypropylene,high-density polyethylene, low-density polyethylene, linear low-densitypolyethylene and co-polymers of this group.
 9. The method of claim 8wherein the first layer is polyester and the other two layers are apolyolefin.
 10. The method of claim 3 wherein the first kind of polymeris polypropylene and the second kinds are ethylene-methyl acrylatecopolymer (EMA).
 11. The method of claim 3 wherein the first kind ofpolymer is polypropylene and the second kinds are ethylene-vinyl acetatecopolymer (EVA).
 12. The method of claim 11 wherein the second polymeris an ionomer resin.